FIG. 1 is a block diagram of a conventional read channel 10 for recovering data from an analog read signal. E.g., a storage-disk read head or a wireless-device antenna (neither shown in FIG. 1) may generate the read signal.
The read channel 10 includes an analog-to-digital converter (ADC) 12, a variable-gain amplifier (VGA) 14, an interpolated-timing-recovery circuit (ITR) 16, a finite-impulse-response filter (FIR) 18, a Viterbi detector 20, and a phase-error detector (PE) 22. The ITR 16, FIR 18, Viterbi detector 20, and PE 22 form a timing loop (TLP) 24 for phase acquisition and tracking. The read channel 10 may also include other components that are omitted from FIG. 1 for brevity.
An example of a read channel similar to the read channel 10 is disclosed in U.S. patent application Ser. No. 11/402,155, filed Apr. 10, 2006, which is incorporated by reference.
The ADC 12 generates raw digital samples S of the read signal in response to a sample clock that is unsynchronized to the read signal, and the VGA 14 adjusts the amplitudes of the raw samples S such that these amplitudes are within a predetermined range that is suitable for the ITR 16, FIR 18, and Viterbi detector 20. Furthermore, although the sample clock is unsynchronized to the read signal, it typically has the same or approximately the same frequency as the data carried by the read signal.
The ITR 16 interpolates the gain-adjusted samples according to a target polynomial (e.g., PR4, EPR4 and E2PR4) that corresponds to the characteristics of the read signal and according to a phase-interpolation value Tau that is related to the phase error between the read signal and the sample clock. Generally, the ITR 16 interpolates each gain-adjusted raw sample by causing the amplitude of the interpolated sample to equal (or approximately equal) the amplitude that the gain-adjusted raw sample would have had if the sample clock were in-phase (phase error=0) with the read signal. That is, the ITR 16 effectively shifts the phase of the interpolated sample by Tau relative to the phase of the gain-adjusted raw sample.
The FIR 18 equalizes the interpolated samples according to the target polynomial. The topology and filter coefficient(s) of the FIR 18 provide the FIR with a transfer function that corresponds to the target polynomial. That is, the transfer function causes the FIR to “fit” the interpolated samples to the target polynomial.
The Viterbi detector 20 recovers data from the read signal according to the target polynomial.
The PE 22 acquires and tracks the phase error between the sample clock and the data carried by the read signal, generates the phase-interpolation value Tau in response to the phase error, and provides the phase-interpolation value Tau to the ITR 16. For example, the PE 22 may generate Tau equal (or approximately equal) to the phase error, although in most instances, Tau does not equal the instantaneous phase error due to the latency of the TLP 24.
Still referring to FIG. 1, a potential problem with the read channel 10 is that the latency of the TLP 24 is relatively long, and thus causes the bandwidth of the loop to be relatively low. Because of this low bandwidth, the TLP 26 may provide little or no correction of high-frequency phase errors caused by, e.g., jitter in the velocity of a rotating storage disk.